Mollies Being Aggressive

[LauraFrog]

I'll try and keep this short, lol...

22 gallon brackish (2'6"). Set up June-ish last year as a freshwater, converted to brackish about October, fully cycled mature tank. It's not too strong, about 3 tsp salt/gallon and some water hardener as well to take the hardness up to about 300ppm.

Current stocking is a bit messy - 7 platys (I can move them if I must to create room for more mollies), 8 rainbows which are 3cm juveniles (getting the salt treatment - recently wild caught) and a young kribensis (had to buy him while he was there, only male I saw in six months of looking.)

The planned stocking is 6-7 bumblebee gobies, and then as many mollies as I can comfortably fit in, adding one or two at a time to keep an eye on the social dynamic. If I've still got heaps of room in the tank, I'll leave the platys there. I added the first pair of sailfins (these are very non-pushy as regards mating, and fine to keep in pairs) in about November or early December. Then I had some issues with parasites, so I stopped adding fish for a month or so to get it cleared up. Just intestinal worms, a few doses of praziquantel has removed them.

The issues are cleared up. Went to the LFS yesterday, and they had this molly... enough said. Sailfin male, one of the nicest I have EVER seen, I might try and get a pic but I doubt it will do him justice. The problem is, the original male is being aggressive and attacking the new one. It isn't serious, he has no injuries, but it is enough to keep him hiding in the plants and discourage him from feeding. At the moment there is one female in the tank, but neither male is bothering her, so I dont' think it's the female that is the problem. The LFS has about fifteen sailfin males in a smaller tank than mine, with apparently no aggression issues. So what should I do? I'm not sure whether I should add more mollies ASAP (male or female?) or just leave things to settle down. I really, REALLY don't want to have to get rid of one of the males, as I don't have another brackish tank big enough (my next biggest is six gallons, guppies/gobies, not a chance!) and there's no way I want to rehome either of them.

Anybody got any advice? Thanks!

 

[Colin_T]

The males are territorial and will fight among themselves. However, they don't normally do a lot of physical damage, it is mostly psychological and one male will dominate the other until it succumbs to disease or leaves the area. In a small tank like your I would suggest only having 1 male sailfin molly and a few females.
For long-term health you don't want too many fish in the tank. You could have some bumblee gobies in with some platies but I wouldn't keep the mollies or rainbows because the gobies won't get any food with them in the tank. Also the mollies and rainbows will want a bit more room when they mature.
A trio of mollies would fine in that tank but preferably not with the BB gobies.

 

[LauraFrog]

The rainbows are not staying in that tank, I know it's too small for them. I just put them in there because it's easier just to chuck them in a brackish tank than muck around trying to temporarily salt up another tank for them. At the moment they are tiny. The kribensis will also be moved because I didn't think it was a good idea to keep him with bbg.

What the LFS has is a 2 foot tank - smaller than mine - with about fifteen adult sailfins of mixed sex in tehre, a few more females than males. They get on fine - no aggression, no harassment of the females by the males to mate - but they are all healthy, active and eating, not standard sick LFS fish. This is a GOOD pet shop. They often stay in there for weeks or months (there's one fish that came in with a missing eye, he's been there for months and is still fine, the others don't even pick on him) and they seem to be fine. What I thought might have happened is that having a larger number of mollies in there prevents them from thinking they can establish a territory, preventing the aggression. That's what I was hoping to recreate, because having a big heap of mollies in a tank certainly worked for them. My tank has exceptional filtration (over 10x/hour - it's running a full size undergravel, a power filter rated for slightly less than its size, and a second power filter rated for up to twice its size. It also gets 50%/week water changes. Do you think there is much chance of recreating this in a home tank? I don't think the turnover of fish is a big issue with the lack of aggression because sailfins are not big sellers. The LFS warns people they will not thrive in freshwater tanks, and they are also expensive compared to most other fish.

As for the bumblebees - if they absolutely will not work in that tank I just won't get them. I've got two at the moment, which are in a 6 gal tank with a trio of guppies. I use a meat baster to get bloodworms down to them past the guppies, and it hasn't been too hard. Would the mollies actually harass them or just make it difficult for them to feed? The two I've got currently are rescues (from a bad pet store keeping them in freshwater and expecting them to eat flake) and in a few weeks they went from lethargic, essentially colourless and refusing to eat to bright black and gold, and trained to eat off the end of a meat baster. This means that if the guppies eat everything I put on the bottom, the gobies can still get food because they eat out of the actual meat baster. It's so funny to watch!

But yeah, basically what I wanted was a few colourful bottom dwellers and if BBG won't work I can try and find something else.

 

[Colin_T]

The mollies probably won't pester the gobies but will try to eat all the food. So unless you can make sure the gobies are being fed and the mollies aren't taking all their food, you would be better off leaving the gobies with the guppies.

The reason the mollies don't fight in the shop is because of the number of fish in the tank. Basically any aggression is distributed among all the fish so no one fish gets continuously picked on.
It is risky having lots of fish in one tank because you only need a slight problem and the entire tank can wipe out. And if you want to breed the mollies then you can't have them crowded. The females stress out when crowded and won't give birth unless they have to. If they do give birth then the other fish eat the fry or the fry are born prematurely and die.
If you want a show tank with male mollies in then get 6-8 males and no females. Put all the males in at the same time and then leave it at that.

 

[LauraFrog]

I'm not really worried about breeding - if I get a few fry that raise themselves in the tank it would be a nice bonus, but I don't have the space to set up enough brackish tanks to raise any number of fry to maturity. The LFS wants them close to full size - I can't raise them to that size in a mixup of five gallon tanks. It's not fair to the fish.

I'm used to overstocking, unfortunately. I had some bad advice when I first started up and I put a ridiculous number of fish in my tanks. I maintained them that way for several months before I could get more tanks. I don't do anything like that now (read as nine platys and a bristlenose in a five gallon tank!) but I still exceed the normal recommendations - for stocking (1.5-2x sometimes) AND filtraton (3 or 4 times) AND water changes (at least double) - in most of my tanks. It seems to work.

I currently have one female but I can possibly rehome her... gotta decide what to do now. The two males I've got at present are starting to get on, the new male has been hiding less today and provided there is no direct challenge for food, the aggression seems to be greatly reduced. Both males are spending most of their time in the open water and there's a lot less nipping. No injuries.